The invention relates to an electrical switchgear apparatus comprising a movable rod and a flexible electrical connector between the movable rod and a fixed connecting strip.
The document EP 0,058,519 describes a switchgear apparatus whose movable contact means comprises a cylindrical rod movable in translation along its axis. This rod is connected to an external connecting strip by means of a flexible conductor formed by a stack of metal strips. Each metal strip has a circular opening provided with flanges protruding out from the plane of the metal strip, towards the center of the opening. To fit the flexible conductor onto the rod, the rod is inserted in the openings of the strip, and the conductors are then sandwiched between two clamping plates. The plates are bolted to one another. When the bolts are tightened, the plates tend to compress the metal strips of the flexible conductor in the area where the openings are located, causing deformation and buckling of the flanges in contact with the rod.
A connection of this kind requires a fairly lengthy assembly time. Moreover, it implements bulky mechanical parts which have to move with the rod when opening and closing of the switchgear apparatus take place. This results in the opening and closing energies being high and in the shocks caused at the end of travel also having a high energy. The whole of the switchgear apparatus opening and closing mechanism then has to be dimensioned so as to be able to supply such opening and closing energies, and to be able to withstand the corresponding shock energies. Furthermore, the temperature of the rod increases when a rated current is flowing through the cartridge, reaching temperatures of around 100xc2x0 C. and more at the level of the flexible conductor fixing. At such temperatures, fixing by bolts does not seem to be a dependable solution in time.
It has been proposed, in U.S. Pat. No. 5,530,216, to fix a flexible electrical connector formed by a stack of metal strips directly to the rod of a vacuum cartridge, without having recourse to clamping plates. The stack comprises two rigid end regions and a flexible middle region. One of the end regions comprises an opening provided with flanges protruding out from the plane of the opening. Once the rod has been inserted in the opening, a press equipped with an annular tool presses the flanges so that the latter constitute a forcible adjustment on the rod. Once the tool has been removed, the rod and the flexible electrical connector are fixedly secured to one another. This fixing process imposes large mechanical stresses on the rod when assembly is performed. However, the movable assembly of a vacuum cartridge is fragile and must not be subjected to stresses outside of its translation axis. Precautions therefore have to be taken to avoid damaging the cartridge when fixing the connector. Furthermore, it should be emphasized that the forcible adjustment zone, which ensures the flow of current between the rod and the movable conductor, has a high resistivity and that it is subjected, when the rated current is flowing, to a temperature increase which tends to cause differential expansions of the different elements, due to the differences of expansion coefficients of the materials constituting the rod on the one hand and the stack of metal plates on the other hand. In addition to this expansion effect, the materials also tend to buckle under the influence of temperature, particularly in the forcible adjustment zone. Moreover, any local deterioration of the joint which may start to appear tends to be accentuated in time, due to the fact that it increases the resistivity locally, causing a temperature increase. The reliability of the fixing is thereby decreased.
A movable contact part for a high-voltage self-extinguishing expansion circuit breaker is moreover described in the document EP 0,932,173. This part comprises a cylindrical conducting tube, one axial end of which bears a contact and the other axial end of which is brazed onto a second conducting element made of die-cast copper. This second conducting element is provided with a base part presenting notches enabling one end of a conducting braid to be welded, the other end of the braid being welded to an electrical connection stud. Such a construction is difficult to transpose to an electrical switchgear apparatus comprising a vacuum cartridge. It is in fact very delicate to weld a braid onto a metal element fixedly secured to the rod of the cartridge, since the thermal energy input is liable to damage the brazes of the cartridge. In addition, the movable contact part obtained is heavy due to the bulky second conducting element. Finally, assembly is relatively complex.
The object of the invention is therefore to remedy the shortcomings of the state of the technique and to propose a junction zone between the movable rod of a switchgear apparatus and a flexible electrical connector, which presents excellent current conducting properties, is light, reliable in time, and simple to manufacture.
According to the invention, this problem is overcome by means of an electrical switchgear apparatus designed to be electrically connected to a busbar and comprising:
a vacuum cartridge comprising a body forming an enclosure housing a pair of separable contacts, one of said contacts being fixedly secured to a metal rod movable in translation, a part of the rod protruding out from the enclosure of the cartridge;
a flexible metallic electrical connector designed to electrically connect the rod to the busbar, the flexible connector being formed by a stack of metal blades and comprising a first rigid end part defining a bored hole in which the protruding part of the rod is inserted;
wherein:
the metal blades are assembled to one another by welding at the level of said first rigid end part so as to form a monoblock assembly at this level,
the flexible electrical connector is fixed to the protruding part of the rod by means of a braze between the protruding part of the rod and the first rigid end part.
The braze achieves a metallurgical connection between the rod and the flexible conductor. The metallic filler compound has a fixed melting point, dependent on the chemical elements which constitute said compound, which is lower than the melting points of the metal parts it enables to be bonded. The parts to be bonded, i.e. the rod and the flexible connector, must not in fact take part by melting in constituting the joint when assembly of the parts is performed. The flexible connector formed by a stack of metal blades assembled to one another by welding at the level of said first rigid end part so as to form a monoblock assembly at this level ensures the good mechanical strength of the assembly, while achieving a great flexibility at low cost.
This fixing mode had not been envisaged up to now, no doubt because it involves a heating stage in the process which was felt to constitute a drawback. The cartridge does in fact itself comprise brazes and materials sensitive to high temperatures. Damage to the cartridge due to the heating stage necessary for brazing the rod and the flexible connector therefore has to be avoided. That is why a metallic filler composition was chosen whose melting point is not very high, i.e., in the context of the present invention, lower than the temperature liable to damage the cartridge. In particular, if the cartridge comprises a sealing bellows brazed onto the rod, a braze having a lower melting temperature than that used to braze the sealing bellows onto the rod will be chosen for the braze fixing the flexible electrical connector to the rod.
To prevent any damage to the cartridge when brazing the electrical connector, the melting point of the braze has to be lower than 900xc2x0 C., and preferably lower than 700xc2x0 C. However, the choice of a braze with a low melting temperature must not be made to the detriment of the electrical conductivity of the braze joint, nor to the detriment of the mechanical strength of this joint. Preferably, the metallic filler compound is a silver-based compound in proportions of more than 30%, with a tin content in proportions of less than 10%. More particularly, proportions of silver greater than 50% with a tin content in proportions of less than 6% will be preferred. The silver gives the braze an excellent electrical conductivity. The tin for its part enables the brazing temperature to be lowered, but has the drawback of making the braze ductile (soft solder), which is undesirable. That is why the proportion of tin has to remain low. Advantageously, a quaternary Agxe2x80x94Cuxe2x80x94Znxe2x80x94Sn compound can be used. According to one embodiment, the compound is quaternary and contains 56% silver, 22% copper, 17% zinc and 5% tin, with a melting point of about 650xc2x0 C. The braze does not contain any cadmium, in spite of the known property of this component to lower brazing temperatures, due to its potential harmfulness for the environment.
The metallurgical connection obtained provides an excellent electrical conduction. It does not increase the weight of the movable assembly, so that the kinetic energy to be dissipated at the end of opening or closing travel remains relatively low. Once brazing has been performed, the joint is not sensitive to the temperatures of around 100xc2x0 C. to which it is subjected when the current is flowing, so that it does not deteriorate in time due to the influence of the temperature.
Advantageously, the protruding part of the rod comprises a spindle, limited on the cartridge side by a shoulder, a part of the filler compound forming an interface joint interposed axially between the shoulder and an axial edge of the bored hole, and another part of the filler compound forming a radial interface joint between the bored hole and the spindle. A joint having an excellent mechanical strength is thus obtained. The spindle constitutes a free end of the rod, so that the assembly operation is particularly simple. All that has to be done is to fit a brazing washer, followed by the bored hole of the connector, onto the spindle, before performing the brazing operation.
Advantageously, the spindle is of circular cross-section, as is the bored hole, so that the radial interface joint between the bored hole and the spindle takes a cylindrical shape, which enables radial positioning constraints of the connector with respect to the spindle to be overcome.
According to one embodiment, welding is performed by electron bombardment welding or by electron scattering welding, without any filler metal.
Advantageously, the flexible connector comprises a second rigid end part forming a connecting strip equipped with means for connection to the busbar, the flexible part being situated between the first and second rigid end parts. The flexible connector then performs the function of both flexible connection and that of connecting strip for a busbar. Bored holes, tapped or not, can be provided for fixing to the busbar.
In this case, it is particularly advantageous to provide for the flexible connector to be formed by a stack of metal blades, assembled to one another by welding at the level of said first and second rigid end parts, the metal blades remaining independent from one another in the flexible part of the connector.